Method for an optical switch on a silicon on insulator substrate

ABSTRACT

Optical cross-connect systems involve the general concept of a two dimensional array of MEMS tilt mirrors being used to direct light coming from a first optical fiber to a second optical fiber. Each MEMS tilt mirror in the two dimensional array can tilt about two non-colinear axes and is suspended by a plurality of suspension arms attached to a silicon on insulator substrate.&lt;/PTEXT&gt;

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention is related to U.S. patent application Ser. No.09/675,945, now issued as U.S. Pat. No. 6,411,427; 09/675,108, nowissued as U.S. Pat. No. 6,466,356; 09/672,380, now issued as U.S. Pat.No. 6,300,665; 09/675,045, now issued as U.S. Pat. No. 6,504,643;09/675,046; 09/672,381, now issued as U.S. Pat. No. 6,501,588; and09/675,093, all filed on the same day and assigned to the same assignee.

BACKGROUND

For optical telecommunication systems it is often necessary to switchthe path of the transmitted light. Numerous different approaches havebeen suggested. Optical switching based on micro-electromechanicalsystem (MEMS) mirrors is particularly attractive for communicationsystems. Optical switches using reflecting MEMS mirrors are convenientbecause free-space light transmission is used and scaling to alarge-scale optical cross-connect system is possible. This is importantbecause of current demand for optical cross-connect systems on the orderof 1000×1000. Actuation to move the MEMS mirrors in an opticalcross-connect system is typically electrostatic, electromagnetic,piezoelectric or thermal.

SUMMARY

Optical cross-connect systems in accordance with an embodiment of theinvention involve the general concept of a two dimensional array of MEMStilt mirrors being used to direct light coming from a first opticalfiber to a second optical fiber. Each MEMS tilt mirror in the twodimensional array can tilt about two non-colinear axes and is suspendedby a plurality of suspension arms attached to a silicon on insulatorsubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment of an n×m optical cross connect system inaccordance with the invention.

FIG. 2a shows an embodiment of a two mirror array optical cross connectsystem in accordance with the invention.

FIG. 2b shows an embodiment of an optical switch system in accordancewith the invention.

FIG. 2c shows an embodiment of an optical switch system in accordancewith the invention.

FIG. 3a shows an embodiment of an n×m optical cross connect system inaccordance with the invention.

FIG. 3b shows dependence of mirror shape on angular incidence.

FIG. 4a shows an embodiment of a tilt mirror structure in accordancewith the invention.

FIG. 4b shows an embodiment of a tilt mirror structure in accordancewith the invention.

FIG. 4c shows an embodiment of a tilt mirror structure in accordancewith the invention.

FIG. 5 shows a side view of an embodiment of a tilt mirror structure inaccordance with an embodiment of the invention.

FIG. 6a shows the effect of mirror curvature on optical beam divergence.

FIG. 6b shows the effect of mirror curvature on optical beam divergence.

FIG. 7a shows a top view of a patterned mask used for processing.

FIG. 7b shows a top view of a patterned mask used for processing.

FIG. 7c shows a top view of a patterned mask used for processing.

FIG. 7d shows a top view of a patterned mask used for processing.

FIG. 7e shows a top view of a patterned mask used for processing.

FIG. 7f shows a top view of a patterned mask used for processing.

FIG. 7g shows a top view of a patterned mask used for processing.

FIG. 7h shows a top view of a patterned mask used for processing.

FIG. 7i shows a top view of a patterned mask used for processing.

FIG. 7j shows a top view of a patterned mask used for processing.

FIG. 7k shows a top view of a patterned mask used for processing.

FIG. 7l shows a top view of a patterned mask used for processing.

FIG. 7m shows a top view of a patterned mask used for processing.

FIGS. 8a-8 o show the processing steps in accordance with an embodimentof the invention.

FIG. 9 shows a top view of an embodiment in accordance with theinvention.

FIG. 10 shows a side view of an embodiment in accordance with theinvention.

FIGS. 11a-11 k show the processing steps in accordance with anembodiment of the invention.

FIG. 12 shows a top view of an embodiment in accordance with theinvention.

FIG. 13 shows a side view of an embodiment in accordance with theinvention.

FIGS. 14a-l show the processing steps in accordance with an embodimentof the invention.

FIG. 15 shows a side view of an embodiment in accordance with theinvention.

FIGS. 16a-16 i show the processing steps in accordance with anembodiment of the invention.

FIGS. 17a-17 l show the processing steps in accordance with anembodiment of the invention.

FIGS. 18a-m show the processing steps in accordance with an embodimentof the invention.

DETAILED DESCRIPTION

FIG. 1 shows an embodiment of optical cross-connect system 100 inaccordance with the invention. Two dimensional array 104 of MEMS tiltmirrors 106 is used to direct light beam 101 coming from two dimensionalarray 108 of optical fibers 110 to two dimensional array 112 of opticalfibers 111. Each mirror 106 can rotate about two non-colinear axes ingeneral. A typical diameter for mirror 106 is in the range of 300 μm to1000 μm. For example, light beam 101 emerging from optical fiber 110 iscollimated using lenslet 115 typically having a diameter greater thanabout 50 μm and projected onto tilt mirror 106 which directs light beam101 onto lenslet 116 which focuses light beam 101 into optical fiber111. Hence, using optical cross-connect system 100, light beam 101coming from any one of optical fibers 110 in two dimensional array 108may be directed by one of mirrors 106 of two dimensional array 104 intoselected optical fiber 111 of two dimensional optical fiber array 112.Note that the number of tilt mirrors 106 is equal to the number ofoptical fibers 110 coming in which in turn is equal to the number ofoptical fibers 111 going out for optical cross-connect system 100.

Light beam 101 in FIG. 1 generally does not enter optical fibers 111head-on and this may lead to aperture issues for optical fibers 111,especially for larger optical fiber arrays or shorter optical pathsresulting in larger scan angles. FIG. 2a shows optical cross-connectsystem 125 in accordance with the invention. Optical cross-connectsystem 125 allows optical fibers 111 to be entered head-on by light beam101. Optical cross-connect system 125 introduces two dimensional array105 of tilt mirrors 107 to assure that light beam 101 enters opticalfiber 111 head-on. Light beam 101 originating from optical fiber 110first strikes tilt mirror 106 and is reflected onto tilt mirror 107which reflects light beam 101 head-on to optical fiber 111. However,optical cross-connect system 125 requires double the number of tiltmirrors that is required using optical cross-connect system 100. Themaximum mirror tilt angle is the maximal angular displacement requiredof mirror 106 or mirror 107 for addressing the most distant mirrors inarray 105 or 104, respectively. Typical maximum mirror tilt angles forthis configuration assuming an optical path length of 8 cm given a beamradius of 180 μm are approximately 3.25° for two dimensional arrays 104and 105.

FIG. 2b shows an embodiment in accordance with the invention for an n×1optical switch for optical circuit switching. Light beam 101 exits fromone of optical fibers 110 in optical fiber array 108 to pass throughlenslet array 115 for collimation and passing to tilt mirror array 104to be reflected by tilt mirror 106 onto tilt mirror 177 which directslight beam 101 through lenslet 117 into desired one of optical fibers114. An application for an n×1 optical switch is to multiplex aselectable subset of m different wavelengths from n (n≧m) differentoptical fibers 110 in array 108 into desired one of optical fibers 114.

FIG. 2c shows an embodiment in accordance with the invention for a 1×noptical switch for optical circuit switching. Light beam 101 exits fromoptical fiber 119 passing through lenslet 118 for collimation and beingreflected by tilt mirror 177 onto desired tilt mirror 107 in tilt mirrorarray 105. Tilt mirror 107 reflects light beam 101 through lenslet array116 for collimation and into desired one of optical fibers 111 inoptical fiber array 112. An application for a 1×n optical switch is toroute optical beam 101 from optical fiber 119 to any one of opticalfibers 111.

Another embodiment in accordance with the invention is shown in FIG. 3a.Optical cross-connect system 150 has two dimensional array 165 of tiltmirrors 106 and 107 and two dimensional array 155 consisting of incomingoptical fibers 110 and outgoing optical fibers 112. In addition, opticalcross-connect system 150 incorporates reflector 180. Light beam 101leaves optical fiber 110 for collimation by lenslet 115 and is reflectedoff of tilt mirror 106 onto reflector 180. From reflector 180 light beam101 is reflected off of tilt mirror 107 into lenslet 115 which focuseslight beam 101 head-on into optical fiber 112.

Mirror shape can be adjusted to be a circle, ellipse or polygon. Forexample, mirror shapes that are elliptical can be used to capture theprojection of a circular beam that is incident at an angle. FIG. 3bshows circular beam 300 incident on ellipsoidal mirror 310 at angle βwith respect to the rays of circular beam 300. An optimum aspect ratiofor ellipsoidal mirror 310 can be derived from angle β.

FIG. 4a shows the basic structure of tilt mirrors 106 and tilt mirrors107 in accordance with an embodiment of the invention. Other geometriesare possible for the tilt mirror structure but the geometry of thealternatives to suspension arms 450 must allow for elongation of thetilt mirror structure between anchor points 440. Otherwise the mirrorstructure cannot raise up after etching of the release layer.

The surface of mirror 405 in FIG. 4a is a substantially flat and stressfree metal to allow precise optical pointing. Mirror 405 is attached byflexure hinges 415 to suspension arms 450. Suspension arms 450 aretypically made of nickel and provide clearance for rotation about axis476 and 475 of mirror 405 by actuation of electrodes 410. Mirror 405 israised automatically during a release etch described below and mirror405 rotates slightly in its own plane as mirror 405 rises from substrate499 (see FIG. 5). Typical heights for mirror 405 having a diameter ofabout 300 to 1000 μm is on the order of 20 to 100 μm. Four actuationelectrodes 410 associated with four suspension arms 450, respectively,may be individually charged to tilt mirror 405 about axis 475 and axis476 with typical actuation voltages being about 10 to 50 volts. Inaddition, electrodes 410 may extend under mirror structure 405 as shownin FIG. 4a.

Actuation electrodes 410 may be actuated using either a DC or AC drive.If AC actuation is used the frequency of the AC drive needs to besignificantly higher than the response time of the mechanical systembeing actuated. AC drive avoids potential buildup of electric charges inthe dielectric materials between or close to actuation electrodes 410.Actuation electrodes 410 are beneficially driven with a bipolar signal,alternating between a positive voltage and an approximately equalnegative voltage. The alternating waveform may typically be squareshaped sinusoidal, triangular or some other suitable shape as long asthe rise and fall times are substantially shorter than the mechanicalresponse time of tilt mirrors 106 and 107, for example.

For the example of a square waveform, a typical drive frequency would behigher than 1 kHz if the mechanical resonance frequency of tilt mirrors106 and 107 were on the order of 1 kHz. Since the actuation force isproportional to the square of the actuation voltage, the actuation forceis independent of the sign of the voltage. The actuation force onlyvaries during the transition from a voltage of one sign to a voltage ofthe opposite sign. Hence, the transition needs to be short compared tothe resonance period of tilt mirrors 106 and 107, for example. Thebipolar signal reduces charge accumulation in the dielectric materialssince the net charge accumulated in the dielectric material averages toapproximately zero. With a DC signal for actuation there is thepossibility of a net charge accumulation in the dielectric material overtime which may act to screen or otherwise interfere with the appliedactuation voltage.

The basic structure shown in FIG. 4a for tilt mirrors 106 and 107 inaccordance with the invention is based on stress-engineered metal films.Mirror 405 and flexure hinges 415 are designed to be stress free whilesuspension arms 450 along the circumference of mirror 405 are made of anickel having an MoCr layer with a built-in stress gradient deposited onthem. Suspension arms 450 are anchored to the substrate at anchor points440. Flexure hinges 415 serve to attach mirror 405 to suspension arms450 while isolating the stress and strain from mirror 405 to maintainplanarity for optical pointing accuracy and providing the rotationalflexibility about axis 477 that is needed for liftup and actuation.

The actuation force for axis 475 and axis 476 is created by theattraction between electrodes 410 situated the substrate and suspensionarms 450. In another embodiment in accordance with the invention,electrodes 410 underlie not only suspension arms 450 but are extendedunder each quadrant of mirror 405 as shown in FIG. 4a to increase thetotal actuation force. However, limiting actuation electrodes 410 to thearea underneath suspension arms 450 provides a larger force per unitarea as the actuation starts from near anchor point 440 where theinitial separation between electrode 410 and suspension arms 450 issmallest and the separation then proceeds to decrease along the lengthof suspension arm 450 in a “zipper-like” fashion as suspension arm 450is drawn toward electrode 410.

Two or more suspension arms 450 may be used in the structure for tiltmirrors 106 and 107 with 4 (four) being typical. If mirror actuation isachieved by actuation of suspension arms 450 alone, a minimum of 3(three) suspension arms 450 is required to allow tilt about twonon-collinear axes. FIG. 4b shows an embodiment in accordance with theinvention of an actuatable mirror structure having 3 (three) suspensionarms 450. If mirror actuation about one tilt axis is achieved by havingelectrodes 410 extend under mirror 405, it is possible to achievetilting about two axes by having only 2 (two) suspension arms 450.

Other embodiments in accordance with the invention are also possible.For example, a requirement for all suspension schemes is that suspensionarms 450 or suspension arms 455 (see FIG. 4c) and/or flexure hinges 415are deformable between anchor points 440. In the embodiments shown inFIGS. 4a and 4 b, the deformation is achieved by using suspension arms450 that wrap around mirror 405. In the embodiment shown in FIG. 4c, thedeformation is achieved through longitudinal flexures 456 in suspensionarms 455. Other embodiments allowing deformation will be readilyapparent to those of ordinary skill in the art.

FIG. 5 shows the basic structure of FIG. 4a in cross-section, showingthe placement of electrodes 410 underneath only suspension arms 450.

Optical path length (defined as the optical distance between exit faceof incoming optical fiber bundle 108 and the entrance face of outgoingoptical fiber bundle 112) effects a number of design parameters. Typicaloptical paths lie in the range of from about 5 cm-10 cm. A longeroptical path for the embodiments shown in FIGS. 1-3 is beneficialbecause it reduces the scan angle required of mirrors 106 and 107 toaddress optical fiber arrays 108 and 112, respectively. A reduced scanangle in turn reduces the actuation voltage required for mirrors 106 and107 to achieve a given resonance frequency or switching speed or resultsin a higher switching speed if the actuation voltage is kept the same. Areduced scan angle also helps lower the mechanical stresses acting onthe flexure elements such as flexure hinges 415 and longitudinalflexures 456. Lowered mechanical stress reduces the potential problemsof metal fatigue or hysteresis.

However, increased optical path length increase the need for opticalbeam collimation. Collimating optics need to be positioned near the exitface of incoming optical fiber bundle 108 and the entrance face ofoutgoing optical fiber bundle 112. This is typically done with lensletarrays 115 and 116 but also with graded index (GRIN) collimators, balllenses or other optical elements suitable for providing collimation.Collimation optics will always leave a finite residual divergence in theoptical bearm, for example, a commercially available GRIN fibercollimator typically leaves a residual divergence angle of from 0.1 to0.25 degrees. The divergence angle combined with the optical path lengthdetermines the size required for mirrors 106 and 107. To avoid intensitylosses, mirrors 106 and 107 need to be larger than the maximum opticalbeam diameter. For a given divergence angle, a longer optical pathrequires larger mirrors 106 and 107 which results in larger mirrorarrays 104, 105 and 165 and a larger array pitch. A larger array pitchagain requires a larger scan angle.

Typical optical beam diameter is about 0.3 mm to 0.5 mm. The beamdiameter after the collimation and expansion optics offers a degree offreedom in design. Expansion of the optical beam diameter relaxes thepositional tolerances of all the optical elements which results insimplified packaging. However, the size required for mirrors 106 and 107is increased. Typical mirror diameters are typically on the order of 300μm-1 mm. As mirror size increase it is more difficult to keep the mirrorsurface optically flat. Increasing mirror thickness enhances the abilityto keep the mirror surface optically flat. A typical thickness formirrors 106 and 107 lies in the range of 1-15 μm.

Mirrors 106 and 107 may have bow which contributes to optical beamdivergence along with residual collimator divergence. FIG. 6a shows across section of concave mirror 600 having radius of curvature R,diameter w, bow x, incoming collimated beam diameter d0, and diameter d2is the diameter of the reflected beam at optical path length L away fromthe surface of concave mirror 600. Bow angle α≈arctan (4×/w) is thedivergence half angle due to bow x. Assuming that bow x is much lessthan radius of curvature R it can be shown that beam diameter d as afunction of optical path length L is given approximately by:

d(L){tilde over ( )}d 0−8xL/w if L<2R  (1)

d(L){tilde over ( )}d 0+8xL/w−2w if L>2R  (2)

and d(L){tilde over ( )}if L{tilde over ( )}R.

FIG. 6b shows that beam diameter as a function of optical path length Lfor convex mirror 610 with optical path length L is given by:

d(L){tilde over ( )}d 0+8xL/w  (3)

From the above it is apparent that the optical beam divergence arisingfrom bow angle a must be kept small with respect to the optical beamdivergence due to the residual collimation angle due to the collimationoptics to maintain acceptable mirror size. For example, if mirrordiameter w is 300 μm , optical path length L is 10 cm and d0 is 250 μm,bow x of 10 nm is acceptable whereas bow x of 100 nm is not acceptable.Increasing mirror diameter w to 500 μm allows box x to be 100 nm.

Larger thickness or diameters for mirrors 106 and 107 mean that mirrors106 and 107 respond more slowly for a given actuation voltage for afixed suspension stiffness and require a higher actuation voltage for afaster response at a higher suspension stiffness.

For example, larger diameters for mirrors 106 and 107 require a higherclearance from the substrate for a given scan angle. The actuation usedfor mirrors 106 and 107 becomes unstable once the downward deflectionexceeds a certain point. The instability typically occurs when the airgap between suspension arms 450 and electrodes 410 decreases to betweenabout 30% to 50% of the air gap for the unactuated state. In theoperation of mirrors 106 and 107 it is desirable to avoid the region ofinstability when operating at the maximum required tilt angle. Toincrease the region of stability, it is beneficial to shape electrodes410 in accordance with an embodiment of the invention. For example,electrodes 410 can be tapered as a function of distance from anchorpoints 440. A typical shape is then a triangular shape. Reducing widthof actuation electrodes 410 along their length gradually reduces theactuation force at a fixed voltage as suspension arms 450 bend downtowards actuation electrodes 410. This reduction in actuation force actsto offset the increased actuation force due to the gradually decreasinggap between suspension arm 450 and actuation electrode 410. Thedecreasing gap is responsible for the onset of the instability.

Hence, the mirror diameter, the required scan angle and the size of theinstability region in combination determine the minimum clearance fromsubstrate 499 for mirrors 106 and 107. Clearance is adjusted byappropriate selection of the length of, for example, suspension arms 450and the magnitude of the stress gradient that is introduced intosuspension arms 450. A typical clearance for mirrors 106 and 107 istypically in the range of 20 μm to 200 μm from substrate 499.

The stiffness of the suspension system and the mass of mirrors 106 and107 determines the resonance frequency of mirrors 106 and 107. Forexample, in FIG. 4a is the stiffness of the suspension system isdetermined by the width, length, thickness and material of suspensionarms 450 and flexural hinges 415. A higher stiffness results in a higherresonance frequency with a resultant higher switching speed but requireshigher actuation voltages. Higher stiffness also reduces the clearanceof mirror 405 over substrate 499 for the same stress gradient andgeometrical configuration. The ratio of the stiffness of suspension arm450 to the stiffness of flexural hinge 415 determines what fraction ofthe actuation force produces the tilt of mirror 405 versus the loweringof mirror 405. Both tilt and lowering of mirror 405 will be present.Hence, it is desirable that the stiffness of flexural hinges 415 is lessthan the stiffness of suspension arms 450.

Two dimensional micro-mirror arrays such as two dimensional array 104 ofMEMS tilt mirrors 106 may be fabricated in a number of ways inaccordance with the invention. Various substrates may be used such as,for example, glass, bulk silicon, and silicon on insulator.

Note that lift-off masks 701-706 In FIGS. 7a-7 f are exemplary and shownfor a two by two micro-mirror array but may be adapted to an arbitrarymicro-mirror array size or to a single mirror structure. FIGS. 8a-8 oshow the fabrication steps for two dimensional array 104 of MEMS tiltmirrors 106 using glass as the substrate in accordance with anembodiment of the invention. FIGS. 8a-8 o are cross-sectional viewstaken substantially along line 8-8 in FIG. 9. High quality unannealedglass substrate 801 is cleaned prior to application of photoresistlift-off mask 701. After application, lift-off mask 701 is patterned asshown in FIG. 7a for anchor points 440, actuation electrodes 410 andelectrical contacts (not shown). Typically, 100 nm of chromium 813 issputter deposited over lift-off mask 701. FIG. 8b shows removal ofphotoresist lift-off mask 701 and the portion of chromium layer 813overlying photoresist lift-off mask 701 using an acetone soak andleaving actuation electrodes 410 (see FIG. 4) and anchor points 440 inplace. Using low pressure chemical vapor deposition (LPCVD), Si₃N₄ layer803 having a typical thickness of about 150 nm is deposited over glasssubstrate 801, over actuation electrodes 410 (see FIG. 4) and anchorpoints 440 as shown in FIG. 8c.

With reference to FIG. 8d, amorphous silicon layer 804 is LPCVDdeposited over Si₃N₄ layer 803 to a typical thickness of about 500 nm.Photoresist mask 702 is applied over amorphous silicon layer 804 andpatterned as shown in FIG. 7b. Vias 890 are dry etched using an O₂/SF₆plasma down to anchor points 440 and electrical contacts (not shown).After removal of photoresist mask 702 using an acetone soak, copper seedlayer 805 is deposited to a typical thickness of about 200 nm overamorphous silicon layer 804, electrical contacts (not shown) and anchorpoints 440 as shown in FIG. 8e.

FIG. 8f shows electroplating mask 703 applied over copper layer 805 andpatterned as shown in FIG. 7c in preparation for electroplating ofnickel layer 806. Nickel layer 806 is electroplated over copper layer805 to a typical thickness of about 1 μm as shown in FIG. 8g. Nickellayer 806 functions as a structural support layer for mirror 405 andsuspension arms 450 (see FIGS. 4 and 9). Electroplating mask 703 is thenremoved. Electroplating mask 704 is applied and patterned as shown inFIG. 7d where only the area for mirror 405 is left exposed.Electroplating mask 704 is the mask for the nickel electroplating ofmirror 405 to a typical thickness of about 2-3 μm as shown in FIG. 8h.Photoresist layer 704 is removed to produce the structure shown in FIG.8i.

MoCr photoresist lift-off mask 705 is applied over mirror 405 and copperlayer 805 as shown in FIG. 8j. Lift-off mask 705 is patterned as shownin FIG. 7e and then MoCr layer 810 is sputter deposited over lift-offmask 705. Typical sputter parameters for deposition of MoCr layer 810are shown in Table 1 below and result in an internal stress gradient ofabout 3.0 Gpa across MoCr layer 810. Total lift can be designed usingconventional micro-spring recipes such as disclosed in U.S. Pat. No.5,914,218 which is incorporated by reference.

TABLE 1 Sputter Deposition Conditions Time (sec) Pressure: 1.6 mT 390Voltage: 518 volts Current: 1.13 A Pressure: 2.2 mT 330 Voltage: 470volts Current: 1.26 A Pressure: 3.0 mT 300 Voltage: 457 volts Current:1.30 A Pressure: 3.9 mT 330 Voltage: 453 volts Current: 1.31 A Pressure:5.0 mT 300 Voltage: 457 volts Current: 1.30 A

An acetone soak is performed to remove lift-off mask 705 and portions ofMoCr layer 810 overlying lift-off mask 705 resulting in the structureshown in FIG. 8k.

Photoresist lift-off mask 706 is applied over exposed copper layer 805,exposed nickel layer 806 and remaining portion of MoCr layer 810.Lift-off mask 706 is patterned as shown in FIG. 7f to expose only thesurface of mirror 405. Gold layer 815 is then sputter deposited to coatmirror 405 with gold. After gold layer 815 has been deposited, lift-offmask 706 along with the portion of gold layer 815 overlying lift-offmask 706 is removed using an acetone soak. The resulting structure isshown in FIG. 8m. To make the structure shown in FIG. 8n, exposed copperlayer 805 is removed using an alkaline etch, typically a mix of5H₂O:5NH₄OH:H₂O₂. This etch avoids damaging the exposed nickel.

Finally, the structure shown in FIG. 8n is released using xenondifluoride (XeF₂) which removes sacrificial amorphous silicon layer 804.Note that remainder of copper layer 805 remains attached to structure899. Removal of amorphous silicon layer 804 causes release of structure899 as shown in FIG. 8o. Structure 899 raises up from substrate 801 dueto the internal stress gradient in MoCr layer 810. Since MoCr layer 810is the surface layer for suspension arms 450 (see also FIG. 4), theinternal stress gradient in MoCr layer 810 acts to force up all four ofsuspension arms 450, thereby raising mirror 405. FIG. 10 shows a partialcut-away view of MEMS tilt mirror 106 on glass substrate 801. FIG. 10shows a partial cut-away view of MEMS tilt mirror 106 on glass substrate801 and showing optical beams 1010 and 1020. Optical beam 1020 reachesmirror 405 by passing through glass substrate 801.

FIGS. 11a-11 k show the fabrication steps for two dimensional array 104of MEMS tilt mirrors 106 using bulk silicon as the substrate inaccordance with an embodiment of the invention. Typical thickness forbulk silicon substrate 1101 is on the order of 100 μm to facilitateetching. FIGS. 11a-11 k are cross-sectional views along line 12-12 inFIG. 12. FIG. 11a shows bulk silicon substrate 1101 with dielectric,typically Si₃N₄, layers 1102 and 1103 deposited on two sides of bulksilicon substrate 1101. Photoresist lift-off mask 709 is applied overdielectric layer 1103 and patterned as shown in FIG. 7i. Cr layer 1105is then sputter deposited over lift-off mask 709 and exposed dielectriclayer 1103. Subsequently, lift-off mask 709 and overlying portions of Crlayer 1105 are removed using an acetone soak.

Following the acetone soak, FIG. 11b shows the deposition of dielectriclayer 1111 over electrodes 410 to electrically isolate electrodes 410.Dielectric layer 1111 may be Si₃N₄ or another dielectric material. SiO₂layer 1106 is deposited over dielectric layer 1111 for release purposes.FIG. 11c shows photoresist mask layer 711 applied over SiO₂ layer 1106and then patterned as shown in FIG. 7k. Exposed portions of dielectriclayer 1111 and SiO₂ layer 1106 are then dry etched away. Photoresistlift-off mask layer 710 is applied in FIG. and patterned as shown inFIG. 7j. As shown in FIG. 11e, MoCr layer 1108 is sputter deposited to atypical thickness of about 500 nm over mask layer 710 as detailed inTable 1 above.

Photoresist lift-off mask 710 and overlying portions of MoCr layer 1108are removed using an acetone soak to achieve the structure in FIG. 11f.FIG. 11g shows photoresist lift-off mask layer 708 applied overdielectric layer 1106 and MoCr layer 1108 and then patterned as shown inFIG. 7h. Gold layer 1109 is sputter deposited over photoresist lift-offmask layer 708 to a typical thickness of about 100 nm. Photolift-offmask layer 708 along with overlying portions of gold layer 1109 are thenremoved using an acetone soak. The entire top of the structure iscovered with photoresist layer 1110 to a thickness of about 5-10 μm asshown in FIG. 11h and hard baked for about 20 minutes at approximately120° C. to protect the top of the structure against subsequentprocessing steps.

With reference to FIG. 11i, photoresist mask 707 is applied todielectric layer 1102 and patterned as shown in FIG. 7g. Photoresistmask 707 exposes the areas for the deep reactive ion etch (DRIE) whichremoves the exposed sections of dielectric layer 1102 and overlying bulksilicon substrate 1101, dielectric layer 1103 and dielectric layer 1106to form suspended mirror 405. FIG. 11j shows the extent of the deepreactive ion etch. MoCr suspension arms 450 (see FIG. 4) are alsoreleased from dielectric layer 1106. Finally, as shown in FIG. 11k,photoresist mask layers 707 and 1110 are removed using either a dry etchor an acetone soak followed by an etch in a photoresist stripper.Finished MEMS tilt mirror structure is shown in FIG. 13 where arrows1310 and 1320 indicate that the bottom as well as the top surface ofmirror 405 may be used to reflect light if minor changes are made to theprocessing steps 11 a-11 j so that the bottom of mirror 405 is alsocoated with gold.

FIGS. 14a-14 l show the fabrication steps for two dimensional array 104of MEMS tilt mirrors 106 using a silicon on insulator substrate inaccordance with an embodiment of the invention. FIGS. 14s-14 l arecross-sectional views along line 12-12 in FIG. 12 . FIG. 14a showssilicon on insulator substrate 1401 with dielectric, typically Si₃N₄,layers 1402 and 1403 deposited on two sides of silicon on insulatorsubstrate 1401. Photoresist lift-off mask 709 is applied over dielectriclayer 1403 and patterned as shown in FIG. 7i. Cr layer 1405 is sputterdeposited to a typical thickness of about 100 nm over patternedphotoresist lift-off mask 709. Photoresist lift-off mask 709 andoverlying portions of Cr layer 1405 are removed using an acetone soak orother standard lift-off technique as shown in FIG. 14b. In FIG. 14c,dielectric layer 1411 is deposited over actuator electrodes 410 toelectrically isolate electrodes 410. Dielectric layer 1411 may be Si₃N₄or another dielectric material. Typically, porous SiO₂ layer 1406 isdeposited over dielectric layer 1411 for release purposes. FIG. 14dshows photoresist mask layer 711 applied over SiO₂ layer 1406 and thenpatterned as shown in FIG. 7k. Exposed portions of SiO₂ layer 1406 andunderlying portions of dielectric layer 1411 are then dry etched away.Photoresist lift-off mask layer 710 is applied over remaining portionsof SiO₂ layer 1406 and exposed portions of layer 141 in FIG. 14e andpatterned as shown in FIG. 7j. As shown in FIG. 14f, MoCr layer 1408 issputter deposited to a typical thickness of about 500 nm over mask layer710 as detailed in Table 1 above.

Photoresist lift-off mask 710 and overlying portions of MoCr layer 1408are removed using an acetone soak or other lift-off technique to achievethe structure shown in FIG. 14g. FIG. 14h shows photoresist lift-offmask 708 applied over dielectric layer 1406 and MoCr layer 1408 and thenpatterned as shown in FIG. 7h. Gold layer 1409 is sputter deposited overphotoresist lift-off mask layer 708 to a typical thickness of about 100nm. Photoresist lift-off mask layer 708 along with overlying portions ofgold layer 1409 are then removed using an acetone soak or other lift-offtechnique. The entire top of the structure is covered with photoresistlayer 1410 to a thickness of about 5-10 nm as shown in FIG. 14i and hardbaked for about 20 minutes at 120° C. to act as protection forsubsequent processing steps. Photoresist mask layer 713 is applied overdielectric layer 1402 and patterned as shown in FIG. 7m. The exposedportion of dielectric layer 1402 is removed using a bufferedhydrofluoric acid etch to allow for the following potassium hydroxideetch. Photoresist mask layer 713 is then also removed using an acetonesoak.

Silicon on insulator substrate 1401 is backside etched using a 45%potassium hydroxide solution at a temperature of approximately 60° C.until buried oxide layer 1475 is reached as shown in FIG. 14j. Buriedoxide layer 1475 functions as an etch stop. Remaining portion ofdielectric layer 1402 and exposed portion of buried dielectric layer1475 are coated with photoresist mask 707 which is patterned as shown inFIG. 7g. Exposed sidewalls in cavity 1450 are also coated withphotoresist layer 1451. The exposed areas are then deep reactive ionetched to remove the exposed portion of buried dielectric layer 1475 aswell as the portions of silicon on insulator substrate 1401, dielectriclayer 1403 and dielectric layer 1406 that lie over exposed portion ofburied dielectric layer 1475. The resultant structure is shown in FIG.14k. Finally, as shown in FIG. 111, photoresist mask layers 707 alongwith photoresist layers 1410 and 1451 are removed using either a dryetch or an acetone soak followed by an etch in a photoresist stripper.Finished MEMS tilt mirror structure 1500 is shown in FIG. 15 wherearrows 1510 and 1520 indicate that the bottom as well as the top surfaceof mirror 405 may be used to reflect light if minor changes are made tothe processing steps 14 a-14 l so that the bottom of mirror 405 iscoated with gold.

FIGS. 16a-16 i show the fabrication steps for two dimensional array 104of MEMS tilt mirrors 106 using any one of the previously mentionedsubstrates by using polysilicon as the mechanical mirror material inaccordance with an embodiment of the invention. FIGS. 16a-16 i arecross-sectional views along line 9-9 in FIG. 9. FIG. 16a showsapplication of photoresist lift-off mask layer 701 on substrate 1601.After application, lift-off mask layer 701 is patterned as shown in FIG.7a for anchor points 440, actuation electrodes 410 and electricalcontacts (not shown). Cr layer 1613 is deposited over lift-off masklayer 701 to a typical thickness of about 100 nm. FIG. 16b shows removalof photoresist lift-off mask layer 701 and the portion of Cr layer 1613overlying photoresist lift-off mask 701 using an acetone soak or otherlift-off technique leaving actuation electrodes 410 (see FIG. 4) andanchor points 440 in place. Using Low Pressure Chemical Vapor Deposition(LPCVD), Si₃N₄ layer 1603 is deposited to a typical thickness of about200 nm over substrate 1601, anchor points 440 and actuation electrodes410 (see FIG. 4) followed by deposition of porous SiO₂ layer 1604 overSi_(3N) ₄ layer 1603 to a typical thickness of about 150 nm as shown inFIG. 16c.

With reference to FIG. 16d, photoresist mask layer 702 is applied toSiO₂ layer 1604 and patterned as shown in FIG. 7b. Vias 1690 are dryetched using an O₂/SF₆ plasma down to anchor points 440 and electricalcontacts (not shown). After removal of photoresist mask layer 702 usingan acetone soak, polysilicon layer 1605 is deposited to a typical depthof about 6 μm to act as the mechanical layer for mirror 405. Then achemical mechanical polish is applied to polysilicon layer 1605 toplanarize the top surface of polysilicon layer 1605 resulting in thestructure shown in FIG. 16e.

Photoresist mask layer 1611 is applied over polysilicon layer 1605 andpatterned as the photo negative of photoresist mask 703 shown in FIG.7c. Exposed portions of polysilicon layer 1605 are then dry etched awaygiving the structure in FIG. 16f. Photoresist mask layer 1611 is thenremoved using either an acetone soak or a dry etch. Photoresist lift-offmask layer 705 is then deposited over exposed SiO₂ layer 1604 andremaining polysilicon layer 1605. With reference to FIG. 16g,photoresist lift-off mask layer 705 is patterned as shown in FIG. 7e andthen MoCr layer 1610 is sputter deposited to a typical thickness ofabout 500 nm on photoresist lift-off mask layer 705 and on exposedportions of polysilicon layer 1605 as described in Table 1. Photoresistlift-off mask layer 705 and overlying portions of MoCr layer 1610 arethen removed using an acetone soak or other lift-off technique.

Photoresist lift-off mask layer 704 is applied over polysilicon layer1605 and exposed portion of SiO₂ layer 1604 and patterned as shown inFIG. 7f. Gold layer 1615 is sputter deposited to a typical thickness ofabout 100nm on photoresist lift-off mask 706 as shown in FIG. 16h. Thenphotoresist lift-off mask 706 and overlying gold layer 1615 is removedusing an acetone soak or other lift-off technique leaving gold layer1615 on mirror 405. Finally, a wet etch is performed on porous SiO₂layer using forty nine percent hydrofluoric acid for about 15 minutes torelease mirror 405 as shown in FIG. 16i. The resulting structure issimilar to that shown in the partial cutaway of FIG. 10.

Tilt mirror 106 flatness can be achieved by making tilt mirrors 106 fromtwo adjacent stress metal layers with opposite stress gradients. FIGS.17a-17 l show the fabrication steps for two dimensional array 104 ofMEMS tilt mirrors 106 using glass as the substrate in accordance with anembodiment of the invention to produce opposing stress. FIGS. 17a-17 mare cross-sectional views taken substantially along line 8-8 in FIG. 9.High quality unannealed glass substrate 801 is cleaned prior toapplication of photoresist lift-off mask 701. After application,lift-off mask 701 is patterned as shown in FIG. 17a for anchor points440, actuation electrodes 410 and electrical contacts (not shown).

Typically, 100 nm of chromium 813 is sputter deposited over lift-offmask 701.

FIG. 17b shows removal of photoresist lift-off mask 701 and the portionof chromium layer 813 overlying photoresist lift-off mask 701 using anacetone soak and leaving actuation electrodes 410 (see FIG. 4) andanchor points 440 in place. Using low pressure chemical vapor deposition(LPCVD), Si₃N₄ layer 803 having a typical thickness of about 150 nm isdeposited over glass substrate 801, over actuation electrodes 410 (seeFIG. 4) and anchor points 440 as shown in FIG. 17c.

With reference to FIG. 17d, amorphous silicon layer 804 is LPCVDdeposited over Si₃N₄ layer 803 to a typical thickness of about 500 nm.Photoresist mask 702 is applied over amorphous silicon layer 804 andpatterned as shown in FIG. 7b. Vias 890 are dry etched using an O₂/SF₆plasma down to anchor points 440 and electrical contacts (not shown).After removal of photoresist mask 702 using an acetone soak, titaniumadhesion layer 1701 is deposited to a typical thickness of about 50 nmover amorphous silicon layer 804, electrical contacts (not shown) andanchor points 440 and followed by deposition of gold reflective layer1705 over titanium adhesion layer 1701 as shown in FIG. 17e.

FIG. 17f shows lift-off mask 703 applied over gold reflective layer 1705and patterned as shown in FIG. 7c in preparation for deposition of MoCrlayer 1710. FIG. 17g shows sputter deposition of five sublayers of MoCr,resulting in a typical total MoCr layer 1712 thickness of 1 μm. Typicalsputter parameters for deposition of MoCr layer 1712 are shown in Table1 above and result in MoCr layer 1712 having an internal stress gradientof about 3.0 Gpa.

Photoresist lift-off mask 704 is applied over MoCr layer 1712. Lift-offmask 704 is patterned as shown in FIG. 7d leaving only the mirror areaexposed. Then MoCr layer 1714 is sputter deposited over lift-off mask704 with a designed in stress gradient opposite to that of MoCr layer1712 as shown in FIG. 17h. This results in substantially zero net forcein mirror 405. An acetone soak is performed to remove lift-off mask 704and portions of MoCr layer 1714 overlying lift-off mask 704. Exposedportion of gold reflective layer 1705 is removed using TRANSENE goldetchant followed by a mixture of HF:H₂O to remove the exposed portion oftitanium adhesion layer 1712 as shown in FIG. 17i.

Photoresist lift-off mask 706 is applied over remaining portion of MoCrlayer 1714 and exposed portion of MoCr layer 1712. Lift-off mask 706 ispatterned as shown in FIG. 7f to expose only the surface of mirror 405.Gold layer 815 is then sputter deposited to coat mirror 405 with gold asshown in FIG. 17j. After gold layer 815 has been deposited, lift-offmask 706 along with the portion of gold layer 815 overlying lift-offmask 706 is removed using an acetone soak. The resulting structure isshown in FIG. 17k. Finally, the structure shown in FIG. 171 is releasedusing xenon difluoride (XeF₂) which removes sacrificial amorphoussilicon layer 804. Removal of amorphous silicon layer 804 causes releaseof structure 1750. Structure 1750 raises up from substrate 801 due tothe internal stress gradient in MoCr layer 1712. Since MoCr layer 1712forms suspension arms 450 (see also FIG. 4), the internal stressgradient in MoCr layer 1712 acts to force up all four of suspension arms450, thereby raising mirror 405.

Tilt mirror 106 flatness can also be achieved by making tilt mirrors 106from two adjacent stress polysilicon layers with opposite stressgradients. Polysilicon can be stressed by adjusting depositiontemperature (as opposed to pressure for MoCr) during LPCVD. Stresses onthe order of 500 mPa can be readily achieved as has been shown by ArthurHeuer of Case Western Reserve University.

FIGS. 18a-18 m show the fabrication steps for MEMS tilt mirrors 106using glass as the substrate in accordance with an embodiment of theinvention to produce opposing stress. FIGS. 18a-18 m are cross-sectionalviews taken substantially along line 8-8 in FIG. 9. High qualityunannealed glass substrate 801 is cleaned prior to application ofphotoresist lift-off mask 701. After application, lift-off mask 701 ispatterned as shown in FIG. 18a for anchor points 440, actuationelectrodes 410 and electrical contacts (not shown). Typically, 100 nm ofchromium 813 is sputter deposited over lift-off mask 701. FIG. 18b showsremoval of photoresist lift-off mask 701 and the portion of chromiumlayer 813 overlying photoresist lift-off mask 701 using an acetone soakand leaving actuation electrodes 410 (see FIG. 4) and anchor points 440in place. Using low pressure chemical vapor deposition (LPCVD), Si₃N₄layer 803 having a typical thickness of about 150 nm is deposited overglass substrate 801, over actuation electrodes 410 (see FIG. 4) andanchor points 440 as shown in FIG. 18c.

With reference to FIG. 18d, amorphous silicon layer 804 is LPCVDdeposited over Si_(3N) ₄ layer 803 to a typical thickness of about 500nm. Then Si₃N₄ layer 1803 is LPCVD deposited over amorphous siliconlayer 804 to serve as a first insulation layer against XeF₂ for thepolysilicon structure to be deposited later. Photoresist mask 702 isapplied over layer Si₃N₄ layer 1803 and patterned as shown in FIG. 7b.Vias 890 are dry etched using an O₂/SF₆ plasma down to anchor points 440and electrical contacts (not shown). After removal of photoresist mask702 using an acetone soak, stressed polysilicon layer 1804 is depositedusing LPCVD and photoresist layer 2004 is applied over polysilicon layer1804 and patterned as the inverse of mask 703 as shown in FIG. 7c.Exposed portion of polysilicon layer 1804 is dry etched in an O₂/SF₆plasma etcher and photoresist layer 2004 is removed resulting in thestructure shown in FIG. 18f.

Photoresist layer 2005 is applied over polysilicon layer 1804 andpatterned as the inverse of mask 703 shown in FIG. 7c but slightlyoptically magnified to produce an approximately 1 μm overhang. Then atimed wet etch using an HF solution is performed of exposed Si₃N₄ layer1803 as shown in FIG. 18g. Photoresist layer 2004 is then removed usingan acetone soak. Si₃N₄ layer 1809 is deposited to encapsulatepolysilicon layer 1804. As shown in FIG. 18h, photoresist mask 704 isdeposited over Si₃N₄ layer 1809 and patterned as shown in FIG. 7d toexpose the mirror region. Si₃N₄ layer 1809 is then etched using an HFetchant.

After removal of photoresist mask 704 using an acetone soak, polysiliconlayer 1805 is deposited with a stress gradient opposite to polysiliconlayer 1804. As shown in FIG. 18i, photoresist layer 2006 is then appliedover polysilicon layer 1805 and patterned to be the inverse of mask 704shown in FIG. 7d. Exposed portions of polysilicon layer 1805 are thendry etched, stopping on Si₃N₄ layer 1809. Photoresist layer 2006 is thenremoved using an acetone soak. As shown in FIG. 18j, photoresist layer2007 is applied over polysilicon layer 1805 and over exposed Si₃N₄ layer1809 and patterned as the inverse of photomask 703 shown in FIG. 7c.Exposed Si₃N₄ layer 1809 is then etched away as shown in FIG. 18j.Photoresist layer 2007 is then removed using an acetone soak.

Photoresist mask 704 is applied over exposed portions of polysiliconlayer 1805, amorphous silicon layer 804 and Si₃N₄ layer 1809 andpatterned as shown in FIG. 7d. Gold layer 1825 is then deposited overphotoresist mask 704 as shown in FIG. 18k. Subsequently, photoresistmask 704 is removed using a liftoff process to leave the structure shownin FIG. 18l. Note that all polysilicon layers are encapsulated againstthe upcoming xenon difluoride etch. Finally, the structure shown in FIG.17m is released using xenon difluoride (XeF₂) which removes sacrificialamorphous silicon layer 804. Removal of amorphous silicon layer 804causes release of structure 1850. Structure 1850 raises up fromsubstrate 801 due to the internal stress gradient in polysilicon layer1803. Since polysilicon layer 1803 forms suspension arms 450 (see alsoFIG. 4), the internal stress gradient in polysilicon layer 1803 acts toforce up all four of suspension arms 450, thereby raising mirror 405.

While the invention has been described in conjunction with specificembodiments, it is evident to those skilled in the art that manyalternatives, modifications, and variations will be apparent in light ofthe foregoing description. Accordingly, the invention is intended toembrace all such alternatives, modifications, and variations that fallwithin the spirit and scope of the appended claims.

What is claimed is:
 1. A method comprising: providing a silicon oninsulator substrate; providing a plurality of metal suspension arms eachcomprising an internal stress gradient layer, each of said plurality ofmetal suspension arms having a first end and a second end, said firstend being attached to said silicon on insulator substrate; providing asilicon area having a reflective surface layer, said silicon area beingattached to said second end of said plurality of metal suspension arms;and providing a plurality of electrodes arranged on said silicon oninsulator substrate adjacent to said plurality of metal suspension armsto create an electric field for producing deflective movement of saidsilicon area having said reflective surface layer.
 2. The method ofclaim 1 wherein said plurality of suspension arms is three in number. 3.The method of claim 1 wherein said silicon area has a substantiallysquare shape.
 4. The method of claim 1 wherein said internal stressgradient layer comprises MoCr.
 5. The method of claim 1 wherein aninternal stress gradient is on the order of 3 gigapascal across saidstress gradient layer.
 6. A method of making an array of structures,each structure including: a silicon area having a reflective surfacelayer; a plurality of metal suspension arms to which the silicon area isattached; and a plurality of electrodes to create an electric field forproducing deflective movement of the silicon area; the silicon area,plurality of metal suspension arms, and plurality of electrodes beingprovided in accordance with claim
 1. 7. The method of claim 6 whereinsaid array is a two dimensional rectilinear array.
 8. A methodcomprising: providing a silicon on insulator substrate; providing fourmetal suspension arms each comprising an internal stress gradient layer,each of said four metal suspension arms having a first end and a secondend, said first end being attached to said silicon on insulatorsubstrate; providing a silicon area having a reflective gold surfacelayer, said silicon area being attached to said second end of said fourmetal suspension arms; and providing four electrodes arranged on saidsilicon on insulator substrate adjacent to said four metal suspensionarms, respectively, to create an electric field for producing deflectivemovement of said silicon area having said reflective gold surface layer.9. A method of making an array of structures, each structure including:a silicon area having a reflective surface layer; four metal suspensionarms to which the silicon area is attached; and four electrodes tocreate an electric field for producing deflective movement of thesilicon area; the silicon area, four metal suspension arms, and fourelectrodes being provided in accordance with claim
 8. 10. The method ofclaim 9 wherein said array is a two dimensional rectilinear array. 11.The method of claim 1 wherein each of said plurality of electrodes has asubstantially tapered shape.
 12. A method comprising: providing asilicon on insulator substrate having a cutout portion; providing aplurality of metal suspension arms each comprising an internal stressgradient layer, each of said plurality of metal suspension arms having afirst end and a second end, said first end being attached to saidsilicon on insulator substrate; providing a silicon area having areflective surface layer and positioned over said cutout portion, saidsilicon area being attached to said second end of said plurality ofmetal suspension arms; and providing a plurality of electrodes arrangedon said substrate adjacent to said plurality of metal suspension arms tocreate an electric field for producing deflective movement of saidsilicon area having said reflective surface layer.
 13. The method ofclaim 12 wherein said reflective surface layer is gold.
 14. A method ofmaking an array of structures, each structure including: a silicon areahaving a reflective surface layer; a plurality of metal suspension armsto which the silicon area is attached; and four electrodes to create anelectric field for producing deflective movement of the silicon area;the silicon area, four metal suspension arms, and four electrodes beingprovided in accordance with claim
 12. 15. The method of claim 14 whereinsaid array is a two dimensional rectilinear array.
 16. The method ofclaim 1 in which each of the metal suspension arms extends away from thesubstrate to its second end, each arm's internal stress gradient layermoving the arm away from the substrate; the plurality of electrodesincluding, for each arm, an electrode that responds to an actuationsignal by attracting the arm toward the substrate.
 17. The method ofclaim 8 in which each of the metal suspension arms extends away from thesubstrate to its second end, each arm's internal stress gradient layermoving the arm away from the substrate; the plurality of electrodesincluding, for each arm, an electrode that responds to an actuationsignal by attracting the arm toward the substrate.
 18. The method ofclaim 12 in which each of the metal suspension arms extends away fromthe substrate to its second end, each arm's internal stress gradientlayer moving the arm away from the substrate; the plurality ofelectrodes including, for each arm, an electrode that responds to anactuation signal by attracting the arm toward the substrate.
 19. Amethod performed on a substrate, comprising: producing an array of atleast one mirror structure at a surface of the substrate, each mirrorstructure including: two or more suspension arms, each arm having aninternal stress gradient, each arm further having an end attached to thesubstrate's surface, the arm extending away from the substrate's surfaceand extending to an attachment point; a reflective area supported on theattachment points of the suspension arms; and adjacent to eachsuspension arm, an electrode on the substrate's surface, the electrodebeing operable to create an electric field to attract the suspension armtoward the substrate's surface; the electrodes adjacent to thesuspension arms of each mirror structure being operable to tilt thereflective area by attracting the suspension arms.
 20. The method ofclaim 19 in which the electrodes of each mirror structure are operableto tilt the reflective area about two non-collinear axes by attractingthe suspension arms.
 21. The method of claim 20 in which each mirrorstructure includes three of the suspension arms.
 22. The method of claim20 in which each mirror structure includes four of the suspension arms.23. The method of claim 19 in which the act of producing the arraycomprises: producing a first patterned layer over the substrate'ssurface, the first patterned layer including the electrodes of themirror structures; producing a second patterned layer over thesubstrate's surface, the second patterned layer including the reflectiveareas of the mirror structures; and producing a third patterned layerover the first patterned layer, the third patterned layer including thesuspension arms of the mirror structures.
 24. The method of claim 23 inwhich the act of producing the third patterned layer comprisesdepositing and patterning a layer with an internal stress gradient, thelayer with the internal stress gradient being present in the suspensionarms.
 25. The method of claim 24 in which the layer with the internalstress gradient includes MoCr or polysilicon.
 26. The method of claim 23in which each mirror structure includes a mirror support for thereflective area, the act of producing the array further comprising:depositing and patterning a mirror support layer, the mirror supportlayer being present in the mirror supports; and the act of producing thesecond patterned layer comprising: depositing and patterning a layer ofgold, the layer of gold being present in the reflective area on themirror support of each mirror structure.
 27. The method of claim 23 inwhich the act of producing the second patterned layer comprises:depositing and patterning a layer of gold over the substrate's surface,the layer of gold being present in the reflective area of each mirrorstructure; the act of producing the array further comprising: backsideetching through the substrate around the reflective area of each mirrorstructure to produce a mirror support for the reflective area.
 28. Themethod of claim 19, further comprising: releasing the mirror structuresto allow the internal stress gradients of the suspension arms to provideclearance between the reflective areas and the substrate's surface. 29.A method performed on a substrate structure, comprising: producing anarray of at least one optical device on the substrate structure, go eachoptical device including: two or more suspension arms, each armextending from a first end attached to the substrate structure to asecond end separated from the substrate structure; each arm having aninternal stress gradient moving the arm away from the substratestructure; an optical element supported on the second ends of thesuspension arms; and for each suspension arm, an electrode on thesubstrate structure, the electrode responding to an actuation signal byattracting the suspension arm toward the substrate structure.
 30. Amethod performed on a substrate structure, comprising: producing anarray of optical devices on the substrate structure, each optical deviceincluding: two or more suspension arms, each arm having a lengthextending from a first end attached to the substrate structure to asecond end that is not attached to the substrate structure; each armbeing movable between a first position in which the arm's length has aninitial separation from the substrate structure and a second position inwhich the arm's length is drawn toward the substrate structure,decreasing separation; each arm having an internal stress gradientmoving the arm's length toward the first position; an optical elementsupported on the second ends of the suspension arms; and for eachsuspension arm, an electrode under the suspension arm, the electroderesponding to an actuation signal by deflecting the suspension arm fromits first position toward its second position; the electrodes respondingindividually to the actuation signals to tilt the optical element aroundtwo non-collinear axes.
 31. A method performed on a silicon on insulatorsubstrate structure, comprising: producing an array of mirror structureson the substrate structure, each mirror structure including: two or moremetal suspension arms, each arm having a length extending from a firstend attached to the substrate structure to a second end that is notattached to the substrate structure; each arm being movable between afirst position in which the arm's length has an initial separation fromthe substrate structure and a second position in which the arm's lengthis drawn toward the substrate structure, decreasing separation; each armhaving an Internal stress gradient moving the arm's length toward thefirst position; a mirror supported on the second ends of the suspensionarms, the mirror including a silicon area and a reflective surfacelayer, and for each suspension arm, an electrode under the suspensionarm, the electrode responding to an actuation signal by creating anelectric field that deflects the suspension arm from its first positiontoward its second position by beginning to decrease separation at thearm's first end and proceeding to decrease separation along the lengthof the arm toward the arm's second end; the electrodes respondingindividually to the actuation signals to tilt the mirror around twonon-collinear axes; the act of producing the array of mirror structurescomprising: producing first, second, and third patterned layers over thesubstrate structure; the first patterned layer including the electrodesof the mirror structures; the second patterned layer being a metal layerover the first patterned layer and including the suspension arms of themirror structures; the third patterned layer including the reflectivesurface layers of the mirrors; and releasing the mirror structures toallow the internal stress gradients of the metal suspension arms to moveeach suspension arm toward its first position.
 32. The method of claim31 which producing the first patterned layer comprises: producing eachelectrode with a shape that is a function of distance from the first endof its suspension arm.
 33. The method of claim 31 in which the act ofproducing the array of mirror structures further comprises, prior toreleasing the mirror structures: backside etching the silicon oninsulator substrate structure to remove portions of the substratestructure around the silicon area.